1. Field of the Invention
The present invention relates to a conductive paste, and more specifically, it relates to a conductive paste used in order to form an electrode, a wiring pattern, etc., made of a thick film on a substrate, and relates to a printed wiring board formed using the same.
2. Description of the Related Art
In general, so called thick film techniques have been widely used in practice wherein conductive pastes are coated on insulating substrates made of glass, ceramics, etc., by a screen printing method or a direct drawing method, and thereafter these are fired to form desired conductive patterns such as electrodes and wiring patterns.
The conductive pastes used in the thick film techniques can be classified, in accordance with firing temperatures, into high temperature firing types which are fired at about 800xc2x0 C. to 950xc2x0 C. and middle temperature firing types which are fired at about 750xc2x0 C. or less, for example, about 600xc2x0 C.
When the high temperature firing type conductive pastes are used, it is possible to obtain electrodes and wiring patterns superior in conductive characteristics, in particular, conductivity and adhesiveness to substrates, while there are drawbacks in that printed resistors and dielectrics are thermally damaged.
On the other hand, the middle temperature firing type conductive pastes have advantages in that these can be fired at lower temperatures and therefore electrodes and wiring patterns can be formed without causing large thermal damage to common printed resistors, dielectrics, etc.
In general, the middle temperature firing type conductive pastes are composed of metallic powders and glass frits dispersed in organic vehicles, wherein the metallic powders are sintered during the firing to be thick film conductors, the glass frits enter the liquid state during the firing to improve the sinterability of metals and to function as adhesives of formed thick film conductors and substrates, and the organic vehicles function as organic liquid media for making these powders possible to print.
The function of the aforementioned glass frits of adhering thick film conductors to substrates is called the glass bonding function. The glass frits melt and move to the interface with the substrates during the firing of the conductive pastes to function as adhesives of fired films to substrates.
Therefore, there are more metallic components in the upper layer part of the thick film conductors after firing, and there are more glass components in the lower layer part. Thus the glass fills the role of a mechanical bonding of the substrates and thick film conductors, wherein the glass is in the shape as if it were reaching out its hand between metallic particles from above the surface of the substrates. In the common middle temperature firing type conductive pastes, in order to simplify the pulverizing process of the glass frits, usually, glass frits of about 3 xcexcm or more in mean diameter are used.
The thick film conductors, however, such as electrodes and wiring patterns, formed using the middle temperature firing type conductive pastes are likely to be inferior in the conductivity characteristics compared to the high temperature firing type conductive pastes.
For example, in the case of the middle temperature firing type conductive pastes using glass frits, the glass frits melt and move to the interface with the substrates during the firing, and pores are formed at the places where the glass frits have been before the firing. Because the pores are formed in accordance with particle diameters of the used glass frits, there are problems in that the conductivity of the obtained thick film conductors may be lowered, the strength of the obtained thick film conductors themselves may be lowered and furthermore, degradation in the tolerance to thermal aging may be caused due to the penetration of solder into the pores.
As the aforementioned middle temperature firing type glass, usually glass containing lead and softening at a low temperature is used. Recent years, however, the environmental pollution by the elution of lead has caused problems, and conductive pastes containing no lead have been required.
The present invention was made in consideration of the aforementioned circumstances. It is an object of the present invention to provide a conductive paste which can form a thick film conductor containing no lead, able to be fired at a middle temperature, superior in the conductivity and having sufficient adhesive strength to a substrate, and to provide a printed wiring board in which an electrode and a wiring pattern are formed using the conductive paste.
In order to attain the aforementioned objects, a conductive paste according to the present invention is a conductive paste comprising: at least one metallic powder selected from the group consisting of Cu, an alloy containing Cu and a mixture containing Cu; a glass frit; and an organic vehicle, and is characterized in that a glass powder which has a composition represented by the formula xBi2O3xe2x80x94yB2O3xe2x80x94zSiO2, where x, y, and z are indicated in mole %, and the composition ratio (x, y, z) of which is within the range surrounded by point A (15, 35, 50), point B (25, 60, 15), point C (45, 40, 15) and point D (45, 20, 35) in a ternary compositional diagram shown in FIG. 1, is used as the glass powder constituting the aforementioned glass frit.
By using the glass frit containing the glass powder which has the composition represented by the formula xBi2O3xe2x80x94yB2O3xe2x80x94zSiO2, and the composition ratio (x, y, z) of which is within the range surrounded by point A, point B, point C and point D shown in FIG. 1 as the glass frit, it becomes possible to lower the firing temperature without using the lead glass, and therefore it becomes possible to provide a conductive paste which can form a thick film conductor containing no lead, being superior in conductivity and having sufficient adhesive strength to a substrate even when it was fired at a middle temperature.
That is, by using the glass powder, the composition ratio (x, y, z) of which is within the range surrounded by the point A, point B, point C and point D shown in FIG. 1 as the glass powder, it becomes possible to make it surely function as the liquid phase preventing the increase of the glass softening point and the crystallization of the glass during the firing of the conductive paste, and it becomes possible to improve the sinterability of a metallic powder, being a conductive material, selected from the group consisting of Cu, an alloy containing Cu and a mixture containing Cu, even when it is fired under a non-oxidative atmosphere. Therefore, it becomes possible to lower the firing temperature without using the lead glass, and it becomes possible to provide a conductive paste which can form the thick film conductor containing Cu as a primary component, superior in the conductivity and having sufficient adhesive strength to the substrate even when it was fired at the middle temperature under the non-oxidative atmosphere.
By using the powder (copper material) selected from the group consisting of Cu, the alloy containing Cu and the mixture containing Cu, it becomes possible to suppress the material cost, to reduce the wiring resistance, and to improve the migration characteristics compared to the case in which noble metals, for example, gold, silver, palladium, etc., are used.
Furthermore, it becomes possible to lower the firing temperature to about 700xc2x0 C. or less, and therefore it becomes possible to reduce the thermal damage to elements such as a resistor formed beforehand on the substrate.
In the conductive paste according to the present invention, a mean particle diameter of the aforementioned glass powder is preferably within the range of about 0.1 to 1.0 xcexcm. By using the glass powder, the mean particle diameter of which is within the range of about 0.1 to 1.0 xcexcm as the glass powder constituting the glass frit, it becomes possible to improve the reactivity with the metallic powder conductive material selected from the group consisting of Cu, the alloy containing Cu and the mixture containing Cu, to improve the sinterability of the metallic powder, and to further lower the firing temperature, and it becomes possible to improve the conductivity of the formed thick film conductor and the strength of the sintered film itself by preventing the formation of large pores in the sintered film, and to suppress the degradation in the tolerance to the thermal aging.
In the conductive paste according to the present invention, the composition ratio (x, y, z) of the aforementioned glass powder represented by the formula xBi2O3xe2x80x94yB2O3xe2x80x94zSiO2 is preferably within the range surrounded by point E (25, 30, 45), point F (25, 55, 20), point G (35, 50, 15) and point H (35, 25, 40) in the ternary compositional diagram shown in FIG. 1.
By using the glass powder, the composition ratio (x, y, z) of which is within the range surrounded by the point E, point F, point G and point H shown in FIG. 1 as the glass powder, it becomes possible to more surely lower the glass softening point, to improve the sinterability of the metallic powder conductive material selected from the group consisting of Cu, the alloy containing Cu and the mixture containing Cu, to obtain a thick film conductor superior in conductivity and having sufficient adhesive strength to the substrate, even when it was fired at the middle temperature under the non-oxidative atmosphere.
In the conductive paste according to the present invention, the aforementioned glass powder preferably contains a positive amount of at least one selected from the group consisting of (a) about 40 mole % or less of ZnO2, (b) about 10 mole % or less of an alkali metal oxide and (c) about 20 mole % or less of an alkaline-earth metal oxide relative to 100 mole % of a primary component which has the composition represented by the formula xBi2O3xe2x80x94yB2O3xe2x80x94zSiO2 and the composition ratio (x, y, z) of which is within the range surrounded by point A (15, 35, 50), point B (25, 60, 15), point C (45, 40, 15) and point D (45, 20, 35) in the ternary compositional diagram shown in FIG. 1.
By using the glass powder containing at least one selected from the group consisting of (a) about 40 mole % or less of ZnO2, (b) about 10 mole % or less of the alkali metal oxide and (c) about 20 mole % or less of the alkaline-earth metal oxide relative to 100 mole % of the primary component, the composition ratio (x, y, z) of which is within the range surrounded by point A, point B, point C and point D in the ternary compositional diagram shown in FIG. 1, it becomes possible to further lower the glass softening point.
In the conductive paste according to the present invention, the content of the aforementioned glass frit is preferably within the range of about 2 to 15 weight % relative to the whole quantity of the aforementioned conductive paste.
By specifying the content of the glass frit containing the aforementioned glass powder as being within the range of about 2 to 15 weight % relative to the whole quantity of the conductive paste, it becomes possible to more surely provide the conductive paste which can form the thick film conductor superior in the conductivity and having sufficient adhesive strength to the substrate even when it was fired at the middle temperature.
The reason the content of the glass frit is specified as being within the aforementioned range are that when the content of the glass flit is about 2 weight % or less, the effect of being the liquid phase may be insufficient so that the sintering at a low temperature is likely not to proceed, and when the content of the glass frit is more than about 15 weight %, the solderability may be degraded.
The firing temperature of the conductive paste of the present invention is preferably about 700xc2x0 C. or less.
By providing with the aforementioned characteristic constitutional factors, it becomes possible to lower the firing temperature to about 700xc2x0 C. or less, it is possible to more surely provide the conductive paste which can surely form the thick film conductor superior in the conductivity and having sufficient adhesive strength to the substrate even when it was fired at the middle temperature under the non-oxidative atmosphere.
The conductive paste according to the present invention is preferably used in order to form an electrode and a wiring pattern of a printed wiring board by printing.
By using the aforementioned conductive paste as the conductive paste in order to form the electrode and the wiring pattern of the printed wiring board by printing, it becomes possible to form a low resistance wiring having sufficient adhesive strength to the substrate and having large film thickness, and it becomes possible to efficiently manufacture the printed wiring board superior in high frequency characteristics.
A printed wiring board according to the present invention is a printed wiring board in which an electrode and a wiring pattern are provided on a substrate, and is characterized in that at least a part of the aforementioned electrode and wiring pattern is formed by coating the conductive paste according to the aforementioned aspects of the present invention and thereafter firing.
In the case in which the electrode and the wiring pattern are formed using the conductive paste according to the present invention, it becomes possible to efficiently form the electrode and the wiring pattern superior in the conductivity and having sufficient adhesive strength to the substrate even when these were fired at the middle temperature, so that it becomes possible to provide printed wiring board low in cost, containing no lead, and being superior in the stability and reliability of the characteristics.
The printed wiring board according to the present invention can be used for functional modules such as a hybrid IC, functional packages, etc., and more specifically, it can be used for single-sided printed wiring substrates, printed wiring boards, multilayer wiring substrates, flexible substrates, ceramic multilayer substrates, etc.